1. Field of the Invention
The present invention generally relates to a method for ensuring data integrity in a disk drive recording system, and more particularly, to a disk drive system incorporating a read-verify after write method to ensure the accuracy of data written to a magnetic disk.
2. Description of the Related Art
Computer disk drive technology evolution has focused on improvements in "areal density", or the number of bits of information that can be stored in a given space on a magnetic disk. Over the last decade, the majority of progress has been gained through miniaturization of the recording heads and improving the magnetic efficiency of the write/read elements in the heads, and similar improvements in the magnetic and physical properties of the disks.
Disk drives contain a plurality of recording heads that "fly" over rotating disks. The magnetic recording efficiency is a function of many physical characteristics of the heads 10 and disks 20, the most significant of which is the spacing between the rotating disk 20 surface and the recording head "pole" elements 30 as shown in FIG. 1. The most straightforward method for manufacturers to improve areal density has been to reduce the spacing between the head and disk, without sacrificing the long term reliability of the disk drive.
Across the previous disk drive industry product offerings, head-disk spacing had steadily decreased from several micro-inches to less than two micro-inches, until there came a point that further increases in areal density required the head to essentially touch the disk during flying. A new class of so-called "pseudo-contact" heads were developed in which the rear portion of the head, where the transducer poles are located, is in constant contact with the disk surface. Various design characteristics were developed to minimize friction and wear between the disk and head, and such "pseudo-contact" designs have proven to be as reliable over the long-term as the non-contact designs.
A problem arises, however, when foreign material or particles come between the head poles and the disk surface, causing a "spacing loss" that affects the magnetic recording process. The types and sources of the foreign material are numerous, including particles generated by mechanical motion of components inside the drive, from contamination introduced at the time of manufacture, to the build-up of excessive lubricant from the disk surface onto the poles of the head. The build-up of contaminants can temporarily increase the spacing between the disk and the recording head, degrading the disk drive performance. Another disadvantage is that contaminants may fall from the recording head during the stop cycles, creating enough friction between the head, the disk and the contaminants that a disk drive motor can no longer move the recording head at the commencement of the start cycle.
Great care is taken to remove sources of contamination during assembly, and in the design and fabrication of components used inside the drive to reduce potential sources of particulate generation. But no volume manufacturing operation can produce completely contamination-free products, and the migration to pseudo-contact recording has significantly increased sensitivity to contamination.
In addition, a positive air pressure with respect to the outside environment is generated inside the head disk assembly ("HDA") as a result of spinning motion of the disks. The motion of air inside the HDA allows for free movement of very small particles and contamination, and due to the large surface area of the disks can result in the material randomly landing on a disk surface. When a recording head is positioned over this area of the disk, the material can displace the head from contacting the disk for as long as it takes this area of the disk to rotate under the head. In most cases, this material remains on the disk, repeatedly displacing the head upon each disk rotation, affecting the magnetic reading or writing function. Otherwise, as a result of repeated contact with the head, the particle is moved to a different location of the disk or is removed from the disk entirely. The presence and movement of this material occurs randomly throughout the life of the disk drive product, and therefore cannot be effectively detected and screened out as part of the manufacturing process.
If this temporary "spacing loss" event occurs during a read function, there are a number of features in conventional disk drives to detect and correct read errors. Some common methods are "retries", where the read is re-attempted, and "ECC Correction", where correction codes appended to the end of the data block are used to determine what the data should have been and then making the correction before transfer to the host computer. Thus although these events are relatively rare, when they occur they often are not detected during read operations.
On the other hand, the writing functions of disk drives do not allow for a methodology of validating that data was written correctly. It is assumed that if a write function is completed successfully, then the write process reliably wrote the magnetic patterns on the disk. However if a temporary spacing loss occurs during a write, it is usually not detected until long after the data was written, at a later time when attempting to read the data. The result is non-recoverable read errors that are reported to the user as lost data. This problem is most commonly known as "High Fly Write", which describes the vertical position of the recording head temporarily being too high during the write process.
In light of the foregoing, there exists a need for a system and method for eliminating or reducing the "High Fly Write" problem, without materially degrading the performance of the disk drive.